Transversely-excited film bulk acoustic resonator fabrication using wafer-to-wafer bonding

ABSTRACT

An acoustic resonator device is formed using a wafer-to-wafer bonding process by etching recesses into a first surface of a piezoelectric substrate, a depth of the recesses greater than a target piezoelectric membrane thickness; then wafer-to-wafer bonding the first surface of the piezoelectric substrate to a handle wafer using a releasable bonding method. The piezoelectric substrate is then thinned to the target piezoelectric membrane thickness to form a piezoelectric plate and at least one conductor pattern is formed on the thinned piezoelectric plate. The side of the thinned piezoelectric plate having the conductor pattern is bonded to a carrier wafer using a metal-to-metal wafer bonding process and the handle wafer is removed.

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

RELATED APPLICATION INFORMATION

This patent is a continuation of copending U.S. application Ser. No.17/546,668, titled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORFABRICATION USING WAFER-TO-WAFER BONDING, filed Dec. 9, 2021 whichclaims priority to co-pending U.S. provisional patent application No.63/175,927, titled METHOD FOR XBAR FABRICATION USING WAFER-TO-WAFERBONDING, filed Apr. 16, 2021, all of which are incorporated herein byreference.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a passband or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies and bandwidths proposed for future communicationsnetworks.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. Radio accesstechnology for mobile telephone networks has been standardized by the3GPP (3^(rd) Generation Partnership Project). Radio access technologyfor 5^(th) generation mobile networks is defined in the 5G NR (newradio) standard. The 5G NR standard defines several new communicationsbands. Two of these new communications bands are n77, which uses thefrequency range from 1300 MHz to 4200 MHz, and n79, which uses thefrequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79use time-division duplexing (TDD), such that a communications deviceoperating in band n77 and/or band n79 use the same frequencies for bothuplink and downlink transmissions. Bandpass filters for bands n77 andn79 must be capable of handling the transmit power of the communicationsdevice. WiFi bands at 5 GHz and 6 GHz also require high frequency andwide bandwidth. The 5G NR standard also defines millimeter wavecommunication bands with frequencies between 24.25 GHz and 40 GHz.

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is anacoustic resonator structure for use in microwave filters. The XBAR isdescribed in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILMBULK ACOUSTIC RESONATOR. An XBAR resonator comprises an interdigitaltransducer (IDT) formed on a thin floating layer, or diaphragm, of asingle-crystal piezoelectric material. The IDT includes a first set ofparallel fingers, extending from a first busbar and a second set ofparallel fingers extending from a second busbar. The first and secondsets of parallel fingers are interleaved. A microwave signal applied tothe IDT excites a shear primary acoustic wave in the piezoelectricdiaphragm. XBAR resonators provide very high electromechanical couplingand high frequency capability. XBAR resonators may be used in a varietyof RF filters including band-reject filters, band-pass filters,duplexers, and multiplexers. XBARs are well suited for use in filtersfor communications bands with frequencies above 3 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view and two schematic cross-sectionalviews of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1.

FIG. 3A is an alternative schematic cross-sectional view of an XBAR.

FIG. 3B is a graphical illustration of the primary acoustic mode ofinterest in an XBAR.

FIG. 3C is a schematic circuit diagram and layout for a high frequencyband-pass filter using XBARs.

FIGS. 4A, 4B and 4C (collectively “FIG. 4”) are a flow chart of aprocess for fabricating an XBAR using wafer-to-wafer bonding.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator orthe same two least significant digits.

DETAILED DESCRIPTION Description of Apparatus

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is a newresonator structure for use in microwave filters. The XBAR is describedin U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILM BULKACOUSTIC RESONATOR, which is incorporated herein by reference in itsentirety. An XBAR resonator comprises a conductor pattern having aninterdigital transducer (IDT) formed on a thin floating layer ordiaphragm of a piezoelectric material. The IDT has two busbars which areeach attached to a set of fingers and the two sets of fingers areinterleaved on the diaphragm over a cavity formed in a substrate uponwhich the resonator is mounted. The diaphragm spans the cavity and mayinclude front-side and/or back-side dielectric layers. A microwavesignal applied to the IDT excites a shear primary acoustic wave in thepiezoelectric diaphragm, such that the acoustic energy flowssubstantially normal to the surfaces of the layer, which is orthogonalor transverse to the direction of the electric field generated by theIDT. XBAR resonators provide very high electromechanical coupling andhigh frequency capability.

A piezoelectric membrane may be a part of a plate of single-crystalpiezoelectric material that spans a cavity in the substrate. Apiezoelectric diaphragm may be the membrane and may include thefront-side and/or back-side dielectric layers. An XBAR resonator may besuch a diaphragm or membrane with an interdigital transducer (IDT)formed on a diaphragm or membrane. Contact pads can be formed atselected locations over the surface of the substrate to provideelectrical connections between the IDT and contact bumps to be attachedto or formed on the contact pads.

XBAR fabrication processes may be divided into two broad categoriesknown as “the front-side etch option” and the “backside etch option”.With the front-side etch option, the piezoelectric plate is attached toa substrate and the active portion of the piezoelectric plate floatsover a cavity (the “swimming pool”) formed by etching away a tub (e.g.,a thickness of an area like a bathtub) of the sacrificial material usingan etchant introduced through holes in the piezoelectric plate. With thebackside etch option, the piezoelectric plate is attached to a substrateand the active portion of the piezoelectric plate floats over a voidetched completely through an area of the substrate and the sacrificialtub from the back side (i.e., the side opposite the piezoelectricplate). The void forms a cavity under the plate.

The following describes improved XBAR resonators, filters andfabrication techniques for XBAR resonators that are fabricated usingwafer-to-wafer bonding, such as by mounting and thinning a piezoelectricsubstrate to form a thinned piezoelectric plate for a wafer-to-waferbonding process that allows a frontside membrane release of the platefrom a substrate when forming a cavity under the plate. This can be doneby etching recesses into a first surface of a piezoelectric substrate toa depth greater than a target piezoelectric membrane thickness, bondingthe first surface of the piezoelectric substrate to a handle wafer usinga releasable bonding method, and then thinning the piezoelectricsubstrate to the target piezoelectric membrane thickness. A conductorpattern is formed on the thinned piezoelectric plate and the side of theplate having the conductor pattern is bonded to a carrier wafer using ametal-to-metal wafer bonding process. The handle wafer is removed so thethinned plate can be bonded to a substrate and a cavity formed under theplate by etching through the recesses.

These improvements avoid the backside etch options or backside membranerelease (BSMR) methods that typically use a deep reactive ion etching(DRIE) technique to remove the Si substrate underneath a piezoelectricplate and the etching time increases with Si substrate thickness.Instead, the technology herein use a frontside membrane release (FSMR)etch processing which takes less etchant and time. Also, other FSMRtechniques use pre-patterned Si substrates with air cavities below theplate and then require frontside device fabrication of conductor layerson thin plates which poses a high risk due to the fragility of the thinplate as the frontside device fabrication can crack and/or damage thethin plate. The technology herein uses an thinning approach thatmitigates the cracking risk by providing the underlying supportstructure of a handle wafer bonded to the plate during the devicefabrication. Thus, the frontside processing to form conductor layers anddielectric layers herein will not crack and/or damage the thin platebecause of the support to the plate by the handle wafer, instead of theplate being suspended over air of a cavity.

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely-excited film bulk acousticresonator (XBAR) 100. XBAR resonators such as the resonator 100 may beused in a variety of RF filters including band-reject filters, band-passfilters, duplexers, and multiplexers. XBARs are particularly suited foruse in filters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. In the examplespresented, the piezoelectric plates may be Z-cut, which is to say the Zaxis is normal to the surfaces. However, XBARs may be fabricated onpiezoelectric plates with other crystallographic orientations.

The back surface 114 of the piezoelectric plate 110 is attached to asubstrate 120 that provides mechanical support to the piezoelectricplate 110. The substrate 120 may be, for example, silicon, sapphire,quartz, or some other material. The substrate may have layers of siliconthermal oxide (TOX) and crystalline silicon. The back surface 114 of thepiezoelectric plate 110 may be bonded to the substrate 120 using a waferbonding process, or grown on the substrate 120, or attached to thesubstrate in some other manner. The piezoelectric plate is attacheddirectly to the substrate or may be attached to the substrate via abonding oxide layer 122, such as a bonding oxide (BOX) layer of SiO2, oranother oxide such as Al2O3.

As shown in FIG. 1, the diaphragm 115 is contiguous with the rest of thepiezoelectric plate 110 around all of a perimeter 145 of the cavity 1.In this context, “contiguous” means “continuously connected without anyintervening item”. However, it is possible for a bonding oxide layer(BOX) to bond the plate 110 to the substrate 120. The BOX layer mayexist between the plate and substrate around perimeter 145 and mayextend further away from the cavity than just within the perimeteritself. In the absence of a process to remove it (i.e., this invention)the BOX is everywhere between the piezoelectric plate and the substrate.The BOX is typically removed from the back of the diaphragm 115 as partof forming the cavity.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers 136 overlap for a distance AP, commonly referred toas the “aperture” of the IDT. The center-to-center distance L betweenthe outermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals orelectrodes of the XBAR 100. A radio frequency or microwave signalapplied between the two busbars 132, 134 of the IDT 130 excites aprimary acoustic mode within the piezoelectric plate 110. As will bediscussed in further detail, the excited primary acoustic mode is a bulkshear mode where acoustic energy propagates along a directionsubstantially orthogonal to the surface of the piezoelectric plate 110,which is also normal, or transverse, to the direction of the electricfield created by the IDT fingers. Thus, the XBAR is considered atransversely-excited film bulk wave resonator.

A cavity 140 is formed in the substrate 120 such that a portion 115 ofthe piezoelectric plate 110 containing the IDT 130 is suspended over thecavity 140 without contacting the substrate 120 or the bottom of thecavity. “Cavity” has its conventional meaning of “an empty space withina solid body.” The cavity may contain a gas, air, or a vacuum. In somecase, there is also a second substrate, package or other material havinga cavity (not shown) above the plate 110, which may be a mirror image ofsubstrate 120 and cavity 140. The cavity above plate 110 may have anempty space depth greater than that of cavity 140. The fingers extendover (and part of the busbars may optionally extend over) the cavity (orbetween the cavities). The cavity 140 may be a hole completely throughthe substrate 120 (as shown in Section A-A and Section B-B of FIG. 1) ora recess in the substrate 120 (as shown subsequently in FIG. 3A). Thecavity 140 may be formed, for example, by selective etching of thesubstrate 120 before or after the piezoelectric plate 110 and thesubstrate 120 are attached. As shown in FIG. 1, the cavity 140 has arectangular shape with an extent greater than the aperture AP and lengthL of the IDT 130. A cavity of an XBAR may have a different shape, suchas a regular or irregular polygon. The cavity of an XBAR may more orfewer than four sides, which may be straight or curved.

The portion 115 of the piezoelectric plate suspended over the cavity 140will be referred to herein as the “diaphragm” (for lack of a betterterm) due to its physical resemblance to the diaphragm of a microphone.The diaphragm may be continuously and seamlessly connected to the restof the piezoelectric plate 110 around all, or nearly all, of perimeterof the cavity 140. In this context, “contiguous” means “continuouslyconnected without any intervening item”. In some cases, a BOX layer maybond the plate 110 to the substrate 120 around the perimeter.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. A typical XBAR has more thanten parallel fingers in the IDT 110. An XBAR may have hundreds, possiblythousands, of parallel fingers in the IDT 110. Similarly, the thicknessof the fingers in the cross-sectional views is greatly exaggerated.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100of FIG. 1. The cross-sectional view may be a portion of the XBAR 100that includes fingers of the IDT. The piezoelectric plate 110 is asingle-crystal layer of piezoelectrical material having a thickness ts.The ts may be, for example, 100 nm to 1500 nm. When used in filters forLTE™ bands from 3.4 GHZ to 6 GHz (e.g., bands 42, 43, 46), the thicknessts may be, for example, 200 nm to 1000 nm.

A front-side dielectric layer 214 may optionally be formed on the frontside of the piezoelectric plate 110. The “front side” of the XBAR is, bydefinition, the surface facing away from the substrate. The front-sidedielectric layer 214 has a thickness tfd. The front-side dielectriclayer 214 is formed between the IDT fingers 236. Although not shown inFIG. 2, the front side dielectric layer 214 may also be deposited overthe IDT fingers 236. A back-side dielectric layer 216 may optionally beformed on the back side of the piezoelectric plate 110. The back-sidedielectric layer may be or include the BOX layer. The back-sidedielectric layer 216 has a thickness tbd. The front-side and back-sidedielectric layers 214, 216 may be a non-piezoelectric dielectricmaterial, such as silicon dioxide or silicon nitride. The tfd and tbdmay be, for example, 0 to 500 nm. tfd and tbd are typically less thanthe thickness ts of the piezoelectric plate. The tfd and tbd are notnecessarily equal, and the front-side and back-side dielectric layers214, 216 are not necessarily the same material. Either or both of thefront-side and back-side dielectric layers 214, 216 may be formed ofmultiple layers of two or more materials.

The front side dielectric layer 214 may be formed over the IDTs of some(e.g., selected ones) of the XBAR devices in a filter. The front sidedielectric 214 may be formed between and cover the IDT finger of someXBAR devices but not be formed on other XBAR devices. For example, afront side frequency-setting dielectric layer may be formed over theIDTs of shunt resonators to lower the resonance frequencies of the shuntresonators with respect to the resonance frequencies of seriesresonators, which have thinner or no front side dielectric. Some filtersmay include two or more different thicknesses of front side dielectricover various resonators. The resonance frequency of the resonators canbe set thus “tuning” the resonator, at least in part, by selecting athicknesses of the front side dielectric.

Further, a passivation layer may be formed over the entire surface ofthe XBAR device 100 except for contact pads where electric connectionsare made to circuitry external to the XBAR device. The passivation layeris a thin dielectric layer intended to seal and protect the surfaces ofthe XBAR device while the XBAR device is incorporated into a package.The front side dielectric layer and/or the passivation layer may be,SiO₂, Si₃N₄, Al₂O₃, some other dielectric material, or a combination ofthese materials.

The thickness of the passivation layer may be selected to protect thepiezoelectric plate and the metal conductors from water and chemicalcorrosion, particularly for power durability purposes. It may range from10 to 100 nm. The passivation material may consist of multiple oxideand/or nitride coatings such as SiO₂ and Si₃N₄ material.

The IDT fingers 236 may be one or more layers of aluminum or asubstantially aluminum alloy, copper or a substantially copper alloy,beryllium, tungsten, molybdenum, gold, or some other conductivematerial. Thin (relative to the total thickness of the conductors)layers of other metals, such as chromium or titanium, may be formedunder and/or over the fingers to improve adhesion between the fingersand the piezoelectric plate 110 and/or to passivate or encapsulate thefingers. The busbars (132, 134 in FIG. 1) of the IDT may be made of thesame or different materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the acoustic wavelength at the resonancefrequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDTis typically close to 0.5 (i.e. the mark or finger width is aboutone-fourth of the acoustic wavelength at resonance). In an XBAR, thepitch p of the IDT is typically 2 to 20 times the width w of thefingers. In addition, the pitch p of the IDT is typically 2 to 20 timesthe thickness is of the piezoelectric slab 212. The width of the IDTfingers in an XBAR is not constrained to one-fourth of the acousticwavelength at resonance. For example, the width of XBAR IDT fingers maybe 500 nm or greater, such that the IDT can be fabricated using opticallithography. The thickness tm of the IDT fingers may be from 100 nm toabout equal to the width w. The thickness of the busbars (132, 134 inFIG. 1) of the IDT may be the same as, or greater than, the thickness tmof the IDT fingers.

FIG. 3A is an alternative cross-sectional view of XBAR device 300 alongthe section plane A-A defined in FIG. 1. In FIG. 3A, a piezoelectricplate 310 is attached to a substrate 320. A portion of the piezoelectricplate 310 forms a diaphragm 315 spanning a cavity 340 in the substrate.The cavity 340, does not fully penetrate the substrate 320, and isformed in the substrate under the portion of the piezoelectric plate 310containing the IDT of an XBAR. Fingers, such as finger 336, of an IDTare disposed on the diaphragm 315. Plate 310, diaphragm 315 and fingers336 may be plate 110, diaphragm 115 and fingers 136. The cavity 340 maybe formed, for example, by etching the substrate 320 before attachingthe piezoelectric plate 310. Alternatively, the cavity 340 may be formedby etching the substrate 320 with a selective etchant that reaches thesubstrate through one or more openings 342 provided in the piezoelectricplate 310. The diaphragm 315 may be contiguous with the rest of thepiezoelectric plate 310 around a large portion of a perimeter 345 of thecavity 340. For example, the diaphragm 315 may be contiguous with therest of the piezoelectric plate 310 around at least 50% of the perimeterof the cavity 340.

One or more intermediate material layers 322 may be attached betweenplate 310 and substrate 320. An intermediary layer may be or include abonding layer, a BOX layer, an etch stop layer, a sealing layer, anadhesive layer or layer of other material that is attached or bonded toplate 310 and substrate 320. Layers 322 may be one or more of any ofthese layers or a combination of these layers.

While the cavity 340 is shown in cross-section, it should be understoodthat the lateral extent of the cavity is a continuous closed band areaof substrate 320 that surrounds and defines the size of the cavity 340in the direction normal to the plane of the drawing. The lateral (i.e.left-right as shown in the figure) extent of the cavity 340 is definedby the lateral edges substrate 320. The vertical (i.e., down from plate310 as shown in the figure) extent or depth of the cavity 340 intosubstrate 320. In this case, the cavity 340 has a side cross-sectionrectangular, or nearly rectangular, cross section.

The XBAR 300 shown in FIG. 3A will be referred to herein as a“front-side etch” configuration since the cavity 340 is etched from thefront side of the substrate 320 (before or after attaching thepiezoelectric plate 310). The XBAR 100 of FIG. 1 will be referred toherein as a “back-side etch” configuration since the cavity 140 isetched from the back side of the substrate 120 after attaching thepiezoelectric plate 110. The XBAR 300 shows one or more openings 342 inthe piezoelectric plate 310 at the left and right sides of the cavity340. However, in some cases openings 342 in the piezoelectric plate 310are only at the left or right side of the cavity 340.

FIG. 3B is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 3B shows a small portion of an XBAR 350including a piezoelectric plate 310 and three interleaved IDT fingers336. XBAR 350 may be part of any XBAR herein. An RF voltage is appliedto the interleaved fingers 336. This voltage creates a time-varyingelectric field between the fingers. The direction of the electric fieldis primarily lateral, or parallel to the surface of the piezoelectricplate 310, as indicated by the arrows labeled “electric field”. Due tothe high dielectric constant of the piezoelectric plate, the electricfield is highly concentrated in the plate relative to the air. Thelateral electric field introduces shear deformation, and thus stronglyexcites a primary shear-mode acoustic mode, in the piezoelectric plate310. In this context, “shear deformation” is defined as deformation inwhich parallel planes in a material remain parallel and maintain aconstant distance while translating relative to each other. A “shearacoustic mode” is defined as an acoustic vibration mode in a medium thatresults in shear deformation of the medium. The shear deformations inthe XBAR 350 are represented by the curves 360, with the adjacent smallarrows providing a schematic indication of the direction and magnitudeof atomic motion. The degree of atomic motion, as well as the thicknessof the piezoelectric plate 310, have been greatly exaggerated for easeof visualization. While the atomic motions are predominantly lateral(i.e. horizontal as shown in FIG. 3B), the direction of acoustic energyflow of the excited primary shear acoustic mode is substantiallyorthogonal to the front and back surface of the piezoelectric plate, asindicated by the arrow 365.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. The piezoelectric coupling for shear waveXBAR resonances can be high (>20%) compared to other acousticresonators. High piezoelectric coupling enables the design andimplementation of microwave and millimeter-wave filters with appreciablebandwidth.

FIG. 3C is a schematic circuit diagram and layout for a high frequencyband-pass filter 370 using XBARs. The filter 370 has a conventionalladder filter architecture including three series resonators 380A, 380B,380C and two shunt resonators 390A, 390B. The three series resonators380A, 380B, and 380C are connected in series between a first port and asecond port. In FIG. 3C, the first and second ports are labeled “In” and“Out”, respectively. However, the filter 370 is bidirectional and eitherport and serve as the input or output of the filter. The two shuntresonators 390A, 390B are connected from nodes between the seriesresonators to ground. All the shunt resonators and series resonators areXBARs on a single die.

The three series resonators 380A, B, C and the two shunt resonators390A, B of the filter 370 are formed on a single plate 310 ofpiezoelectric material bonded to a silicon substrate (not visible). Eachresonator includes a respective IDT (not shown), with at least thefingers of the IDT disposed over a cavity in the substrate. In this andsimilar contexts, the term “respective” means “relating things each toeach”, which is to say with a one-to-one correspondence. In FIG. 3C, thecavities are illustrated schematically as the dashed rectangles (such asthe rectangle 345). In this example, each IDT is disposed over arespective cavity. In other filters, the IDTs of two or more resonatorsmay be disposed over a single cavity.

Description of Methods

FIGS. 4A, 4B and 4C (collectively “FIG. 4”) are a flow chart 400 of aprocess for fabricating an XBAR or a filter incorporating XBARs usingwafer-to-wafer bonding. The process 400 includes mounting and thinning apiezoelectric substrate to form a piezoelectric plate for awafer-to-wafer bonding process of the plate to a substrate for frontsidemembrane release of the plate when a cavity is etched in the substrate.It may include an piezoelectric LiNbO3 (LN) substrate thinning method tothin the substrate to form an LN plate for the wafer-to-wafer bonding.The process 400 is or is included in the forming of XBAR 100, 300, 350and/or of filter 370.

The flow chart of FIG. 4 includes only major process steps. Variousconventional process steps (e.g. surface preparation, chemicalmechanical processing (CMP), cleaning, inspection, deposition,photolithography, baking, annealing, monitoring, testing, etc.) may beperformed before, between, after, and during the steps shown in FIG. 4.

The process 400 starts at 411 with obtaining or receiving a thickpiezoelectric substrate, of material noted for plate 110 and/or 310. Theprocess 400 ends at 475 with a completed XBAR or filter 370. Thepiezoelectric substrate may be, for example, Z-cut, rotated Z-cut, orrotated Y-cut lithium niobate or lithium tantalate. The piezoelectricsubstrate may be some other material and/or some other cut as previouslynoted for plate 110. The substrate may be an LiNbO3 (LN) substratehaving a thickness between 150 um and 350 um. It may have a thickness of250 um. At 611, the plate substrate may be received such as by beingobtained or purchased from an outside source.

After 411, at 415, recesses 418 are etched into a first surface 413 ofthe piezoelectric substrate obtained at 411 to a depth tr greater than atarget piezoelectric membrane thickness ts to form device 401. Thicknessts may be a target thickness for plate 310 after thinning ofpiezoelectric substrate 415 at step 435. Depth tr may be between 100 nmand 500 nm. It may be between 250 nm and 300 nm. It may be 275 nm. Insome cases, thickness ts is between 100 nm and 1000 nm; depth tr isbetween 200 nm and 1500 nm; and depth tr is greater than thickness ts.

Recesses 418 are formed in the piezoelectric substrate in the locationswhere the cavity 140 or 340 is desired to form device 402. Referring toFIG. 3A, the recesses may be locations in the for one or more openings342 provided in the piezoelectric plate 310 through which the cavity 340will be formed by etching the substrate 320 with a selective etchantthat reaches the substrate through these openings 342. While therecesses 418 are only shown in cross-section in FIG. 4, it must beunderstood that each recess 418 is a three-dimensional opening createdby removing material from the obtained piezoelectric substrate of step411 to form plate wafer 412 or device 401.

Recess 418 may have a cross-sectional shape (normal to the plane of thedrawing) that is a rectangle, a regular or irregular polygon, oval, orsome other shape desired for etching through to form cavity 340. Therecess 418 may be formed by etching the plate wafer through a suitablemask such as a photoresist mask or a hard mask. The plate may beselectively etched with respect to the mask. The recesses 418 may beetched into the plate wafer using a suitable wet or dry etching process.The recesses may be formed by a timed etch. For example, the recessesmay be formed using ion milling. In some cases, the etching may be doneby a reactive ion etching (RIE), an inductively coupled plasma (ICP)and/or a laser milling process. Other etching processes may be used onthe plate wafer.

At 425, the first surface 413 of the piezoelectric plate wafer 412 iswafer-to-wafer bonded to a handle wafer 484 using a releasable bondingmaterial 482 to form device 402. Here, the recesses are facing thebonding material 482. The piezoelectric plate wafer 412 and the handlewafer 484 may be wafer-to-wafer bonded by a wafer bonding process thatuses releasable bonding material 482 having a surface 414 bonded tosurface 413 of plate wafer 412. Intervening layers may exist between thereleasable bonding material 482 and the handle wafer 484. The matingsurface 413 of the piezoelectric plate wafer 412 may be highly polished.In some cases, a range of thickness of handle wafer 484 is a thicknessof between 250 um and 1000 um.

The handle wafer 484 may be or include silicon, quartz, glass and/orsapphire. The releasable bonding material 482 may be or include organicadhesives, inorganic adhesives and/or other bonding adhesives. Foradhesive bonding, a huge variety of organic adhesives (mainly polymers)with different chemistries and material properties can be used forbonding material 482. For example, 3M® special UV-curable adhesives,epoxies, and/or benzocyclobutene (BCB) can be used as bonding material482. Inorganic adhesives that can be used as bonding material 482 aremostly ceramic materials that are based on oxides or silicates. Thebonding material 482 can also be a bi-layer system comprised of alaser-releasable layer (LRL) and an adhesive. The bonding process at 425may include fusion bonding, anodic bonding, surface-activated bonding,metal layer bonding, thermocompression bonding, and insulatinginterlayer adhesive and glass frit bonding.

At 435, the piezoelectric plate 412 is thinned on the side or surface416 away from the handle wafer 484 until the plate 412 is the targetpiezoelectric membrane thickness ts, to form device 403. Thus, plate 412becomes plate 310 having surface 417; and the recesses 418 extendcompletely through the thickness ts of the plate 310 to create holes oropenings 342. Planarizing the plate 412 may include thinning the plateby ion slicing, chemical mechanical polishing (CMP) or some other methodto polish the plate to a desired thickness ts.

Thinning at 435 may include grinding and polishing the plate to targetthickness ts where RA<1 nm, and TTV+/−10 nm, where RA is the RoughnessAverage of a surface's measured microscopic peaks and valleys and TTV isthe Total Thickness Variation across the wafer. The recesses 418 and/oropenings 342 will allow interconnection of the IDT (e.g., busbars) oflayer 330 to the gold pads of an M2 metal layer 470 for electricalcontacts on the carrier wafer for the wafer-to-wafer bonding structures,such as at 455. The recesses 418 and/or openings 342 can also be usedfor frontside etching through to form the cavity 340 under the membraneor plate 310. For frontside cavity etching, the openings 342 can be usedas access points for the etchant, such as to layer 490.

At 445, a conductor pattern is formed on surface 417 of the thinnedpiezoelectric plate 310 that is away from the handle wafer 484 to formdevice 404. The conductor pattern includes an IDT 130 having interleavedfingers 336. The conductor pattern may be an M1 metal layer.

A mask may be patterned onto the top surface 417 of plate 310 to formthe IDT 130. Forming IDT may include forming conductor patterns anddielectric layers defining one or more XBAR devices on the surface ofthe piezoelectric plate 310. Typically, a filter device will have an IDTas a first of two or more conductor layers that are sequentiallydeposited and patterned. The IDT layers may be, for example, aluminum,an aluminum alloy, copper, a copper alloy, molybdenum, tungsten,beryllium, gold, or some other conductive metal. Optionally, one or morelayers of other materials may be disposed below (i.e. between the IDTlayer and the piezoelectric plate) and/or on top of the IDT. Forexample, a thin film of titanium, chrome, or other metal may be used toimprove the adhesion between the IDT layer and the piezoelectric plate.

The IDT 130 may be formed by depositing the conductor layers over thesurface 417 of the piezoelectric plate 310 and removing excess metal byetching through a patterned photoresist that covers areas of the IDT.Alternatively, the IDT may be formed using a lift-off process.Photoresist may be deposited over the piezoelectric plate and patternedto remove areas that leave behind or define the IDT. The IDT materialmay be deposited in sequence over the surface of the photoresist andpiezoelectric plate. The photoresist may then be removed, which removesthe excess material, leaving the IDT.

Also, at 445, a second M2 metal layer 470 may be formed on the conductorpattern and/or a passivation layer 419 may be formed on part of theconductor pattern, such as over the diaphragm 315 to be formed later.

Second M2 metal layer 470 is electrically conductive material attachedto the top of the M1 layers or IDT, such as to the top of the busbarsand not to the top of the fingers 336. In some cases, the second metallayers 470 are multiple conductive layers formed similar to forming theIDT 130.

Layer 470 may include contact pads for connecting to contact bumps. Thecontact pads may be for electrically contacting contact bumps that arebonding pads, gold or solder bumps, or other means for making connectionbetween the device 404 (e.g., contact pads, conductor layers or busbars)and carrier wafer 490 at step 455; and/or between device 404 and apackage, PCB and/or external circuitry, at step 475.

The material of layer M1 and layer M2 may be a metal or conductor asdescribed for IDT 130. They may be the same material. They may be adifferent material. They may be formed during one or more differentprocessing steps. These steps may be different than steps for formingthe IDT.

Forming passivation layer 419 may include forming one or more dielectriclayers on the plate and/or IDT, such as a front side dielectric and/or apassivation layer as noted herein. The passivation layer 419 mayinclude, for example, a dielectric layer selectively formed over theIDTs of shunt resonators to shift the resonance frequency of the shuntresonators relative to the resonance frequency of series resonators asdescribed in U.S. Pat. No. 10,491,192. The different thickness of thedielectric layer may cause the XBAR to be tuned to a selected frequencyas compared to the other XBARs. For example, the resonance frequenciesof the XBARs in a filter may be tuned using different front-sidedielectric layer thickness on some XBARs as compared to others. Thepassivation layer 419 may include an encapsulation/passivation layerdeposited over all or a membrane area of the device. Forming passivationlayer 419 may include, for example, depositing anencapsulation/passivation layer such as SiO₂ or Si₃N₄ over a portion ofthe device 403.

Forming the M1, M2 and passivation layer may each include patterning andfabricating those layers separately. In some cases, the plate 310 isLithium Niobate (LN) which may be 275 nm thick; the passivation layer419 is SiO2; the M1 or IDT is Aluminum (Al) metal traces; and the M2 isGold (Au) contact pads and traces.

At 455, the side of the plate 310 having the conductor pattern and IDT130 is wafer-to-wafer bonded to a carrier wafer 490 using ametal-to-metal wafer bonding process to form device 405. At 455, topsurfaces 474 of M2 layer 470 are metal-to-metal bonded to top surfaces494 of metal layer 492 of carrier wafer 490. Surfaces 474 and 494 may bepolished or otherwise treated conductor or metal surfaces of anintegrated circuit (IC) chip or transistor (e.g., a field electrictransistor—FET). Metal layer 492 includes contact pads 496 forconnecting to contact bumps. The contact pads may be for electricallycontacting contact bumps that are bonding pads, gold or solder bumps, orother means for making connection between the device 405 (e.g., contactpads, conductor layers or busbars) and a package, PCB and/or externalcircuitry, at step 475.

Carrier wafer 490 may be or include a material as noted for handle wafer484. Carrier wafer 490 may be a silicon carrier wafer with patternedstructures 492. It may be a printed circuit board (PCB) and/or a packagefor the XBAR or device 404. It may be: 1) mounted above and onto the topsurfaces 474 of the M2 layer 470; and/or 2) mounted using contact bumpsbonded between surfaces 494 of wafer 490 and surfaces 474 of M2 layer470. In some cases, it is only mounted using 1) or 2) above. Wafer 490may be or contain a printed circuit board (PCB) that includes metaltraces. It may be formed by high-temperature co-fired ceramics (HTCC)with signal routing (e.g., vias, traces and contact pads). In somecases, the package is a PCB laminate with copper (Cu) signal routing. Itmay be formed by known PCB processes and have known signal routing.

The metal-to-metal bonding may be done by chemical, atomic and/oradhesive bonding. It may be attaching surfaces 474 and 494 by beingchemically or adhesively bonded to each other; or by being chemicallyand/or thermally treated at those surfaces to cause them to bond.Thermal annealing may be involved. The bonding process at 455 mayinclude eutectic bonding, solder bonding, and/or thermocompressionbonding.

Bonding at 455 forms air cavity 440 between the plate 310 and thecarrier wafer 490 as also shown as step 465. Cavity 440 is formed suchthat a diaphragm 315 of the piezoelectric plate 310 containing the IDT330 is suspended over the cavity 440 without contacting the wafer 490 orthe bottom of the cavity.

At 465, the handle wafer 484 is removed from the plate 310 by removingthe releasable bonding material 482 to form device 406. Removing thehandle wafer releases plate 310 from wafer 484 and forms diaphragm 315over cavity 440. Device 406 may represent device 100, 200 and/or 300.Cavity 440 may have a shape and depth as noted for cavity 140 or 340FIGS. 1-3. The diaphragm 315 may be contiguous with the rest of thepiezoelectric plate 310 around a large portion of a perimeter 345 of thecavity 440. A separate cavity may be formed for each resonator in afilter device. Typically, the cavities 440 formed at 455 may notpenetrate through wafer 490, and the resulting resonator devices willhave a cross-section as shown at step 465.

The releasable bonding material 482 may be removed, for example, bymaterial-dependent wet or dry etching or some other process. Thedependent etch may be selective to the plate 310, conductor layer or IDT130; M2 layer 270 layer 492 and wafer 490 while etching material 482.The etch may or may not etch handle wafer 484 while removing layer 482.In some cases, the etch may simply be a wash that cleans material 482from the surfaces of wafer 414 and plate 310 without removing any of theother components or layers of device 405. In cases where the bondingmaterial is a bi-layer system comprised of a laser-releasable layer(LRL) and an adhesive; a laser can be used to release the LRL attachedto the handle wafer, followed by a subsequent rinse of the adhesivelayer.

The filter or XBAR device 406 may be completed at 465. Actions that mayoccur at 465 include excising individual devices from a wafer containingmultiple devices; other packaging steps; and testing. Another actionthat may occur at 465 is to tune the resonant frequencies of theresonators within a filter device.

At 475, contact bumps 480 are connected between top surfaces of contactpads 496 and contacts pads 498 of capping layer 499 to form device 407.At 475, device 406 may have gold stud bumps 480 attached to pads 496then be flip-chipped bonded to a PCB or ceramic substrate 499, then astandard overmolding process (not shown) may be used to encapsulate thedevice 407. Contact bumps 480 may be first formed on pads 496 thenbonded to pads 498; or vice versa. The contact bumps electricallyconnect pads 496 to pads 498. The contact bumps 480 are bonding pads,gold or solder bumps, and/or other means for making electricalconnections.

Capping layer 499 may be or include a material as noted for handle wafer484. Capping layer 499 may be a silicon carrier wafer with patternedstructures including contacts 498. It may be a printed circuit board(PCB) and/or a package for the XBAR or device 406. It may be: 1) mountedabove and onto the top surfaces 413 of device 406; and/or 2) mountedusing contact bumps 480 bonded between the pads 496 and 498. Cappinglayer 499 may be or contain a printed circuit board (PCB) that includesmetal traces. It may be formed by high-temperature co-fired ceramics(HTCC) with signal routing (e.g., vias, traces and contact pads). Insome cases, the package is a PCB laminate with copper (Cu) signalrouting. It may be formed by known PCB processes and have known signalrouting. Capping layer 499 may be or include a protective encapsulatinglayer for the device 406.

Connecting contact bumps 480 may be metal-to-metal bonding done bychemical, atomic and/or adhesive bonding. It may be thermally treatingthe metal surfaces of the bumps and pads to cause them to bond. Thermalannealing may be involved. The bonding process at 475 may includeeutectic bonding, solder bonding, and/or thermocompression bonding.

Electrical connections may be made between the filter device 406 andexternal devices through vias though the carrier wafer 490 that areconnected between the contact pads 496 and parts of layer 492 that areconnected to layer 470. Vias through the backside of the carrier wafer490 are also possible to connect between the contact pads on the back ofwafer 490 and parts of layer 492 that are connected to layer 470.

The frequency of the filter device 406 may be trimmed by electricallytesting the filter (using the vias in the carrier wafer 490) prior tostep 475 and then tuning device 406 by selectively adding or removingmaterial from the exposed surface of the thinned plate 310. Trimming canbe done on the backside surface of the thinned plate 310 of device 406.After filter electrical testing, the LN thinned plate 310 can be iontrimmed on its back surface 413 to reduce thickness to increase filterfrequency. If the filter frequency is high, then a thin layer of SiO2 orSi3N4 can be deposited on surface 413 to reduce frequency.

Thus, process 400 forms improved XBAR resonators or filters usingfabrication techniques having the wafer-to-wafer bonding process ofsteps 425 and 455 (and optionally 475), such as by mounting and thinninga piezoelectric substrate of a plate wafer 484 for the wafer-to-waferbonding process. This process may have advantages of a frontsidemembrane release of the plate 310 from a substrate 320 when forming acavity 340 under the plate.

Problems solved by process 400 use a wafer-to-wafer bonding process toprovide a lower cost, more controllable approach for forming XBARdevices as compared to a backside membrane release (BSMR) process. Thedescribed piezoelectric substrate thinning method of process 400 willenable frontside device fabrication on thin piezoelectric plates at 445prior to wafer-to-wafer bonding at 455. For instance, a BSMR method mayuse a deep reactive ion etching technique to remove the Si substrate 120underneath the piezoelectric plate 110 and the etching time increaseswith Si substrate thickness. The FSMR of process 400 eliminates the BSMRetch processing which eliminates taking more etching chemistry, etchantand/or time of the BSMR.

It is also noted that other FSMR techniques on pre-patterned Sisubstrates utilize air cavities below and require frontside devicefabrication on thin piezoelectric plates which poses a high risk due tothe fragility of the thin plate because the frontside processing cancrack and/or damage the thin plate. The process 400 uses a piezoelectricsubstrate thinning approach at step 435 that mitigates this crackingrisk by providing an underlying support structure of handle wafer 484for the piezoelectric plate during the device processing as shown. Thus,the frontside processing to form conductor layers and dielectric layersat 445 will not crack and/or damage the thin plate 310 because thehandle wafer 484 will support the plate 310, instead of the plate beingsuspended over air or a layer that is not a thick handle wafer.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An acoustic resonator device comprising: a carrierwafer having a carrier surface and a carrier conductor pattern formed onthe carrier surface; a single-crystal piezoelectric plate having frontand back surfaces, a portion of the piezoelectric plate forming adiaphragm that spans a cavity in the carrier wafer; a device conductorpattern including an interdigital transducer (IDT) formed on the frontsurface of the single-crystal piezoelectric plate and facing the carrierwafer, wherein interleaved fingers of the IDT are disposed on thediaphragm, the overlapping distance of the interleaved fingers definingan aperture of the resonator device; and a second metal layer atselected locations over a surface of the device conductor pattern toprovide electrical connections between the IDT and the carrier conductorpattern, wherein the surfaces of the second metal layer aremetal-to-metal bonded to surfaces of the carrier conductor pattern. 2.The device of claim 1, wherein the piezoelectric plate comprisesopenings extending through the piezoelectric plate.
 3. The device ofclaim 2 further comprising contact pads of the carrier wafer bonded bycontact bumps extending through the openings extending through thepiezoelectric plate to contact pads of a capping layer.
 4. The device ofclaim 1, wherein the device conductor pattern comprises a passivationlayer over the interleaved fingers of the conductor pattern.
 5. Thedevice of claim 1, wherein: the piezoelectric plate and the deviceconductor pattern are configured such that radio frequency signalsapplied to the device conductor pattern excites a primary shear acousticmode in the piezoelectric plate over the cavity, wherein a thickness ofthe diaphragm is selected to tune the primary shear acoustic modes inthe piezoelectric plate.
 6. The device of claim 1, wherein a thicknessof the plate ts is between 100 nm and 1000 nms.
 7. The device of claim1, wherein the interleaved fingers are disposed over the cavity.
 8. Anacoustic resonator device comprising: a carrier wafer having a carriersurface and a carrier conductor pattern formed on the carrier surface; asingle-crystal piezoelectric plate having front and back surfaces, aportion of the piezoelectric plate forming a diaphragm that spans acavity in the carrier wafer, the cavity formed between the diaphragm,the carrier conductor pattern and the carrier surface; a deviceconductor pattern including an interdigital transducer (IDT) formed onthe front surface of the single-crystal piezoelectric plate, whereininterleaved fingers of the IDT are disposed on the diaphragm and towardsthe carrier wafer, the overlapping distance of the interleaved fingersdefining an aperture of the resonator device; and a second metal layerat selected locations over a surface of the device conductor pattern,wherein the surfaces of the second metal layer are metal-to-metal bondedto surfaces of the carrier conductor pattern.
 9. The device of claim 8,wherein the piezoelectric plate comprises openings extending through thepiezoelectric plate.
 10. The device of claim 9 further comprisingcontact pads of the carrier wafer bonded by contact bumps extendingthrough the openings extending through the piezoelectric plate tocontact pads of a capping layer.
 11. The device of claim 8, wherein theinterleaved fingers are disposed towards the cavity, and wherein thedevice conductor pattern comprises a passivation layer over theinterleaved fingers of the conductor pattern and disposed over thecavity.
 12. The device of claim 8, wherein: the piezoelectric plate andthe device conductor pattern are configured such that radio frequencysignals applied to the device conductor pattern excites a primary shearacoustic mode in the piezoelectric plate over the cavity, wherein athickness of the diaphragm is selected to tune the primary shearacoustic modes in the piezoelectric plate.
 13. The device of claim 8,wherein a thickness of the plate ts is between 100 nm and 1000 nms. 14.An acoustic resonator device comprising: a carrier wafer having acarrier surface and a carrier metal layer over the carrier surface; apiezoelectric plate having front and back surfaces, a portion of thepiezoelectric plate forming a diaphragm that spans a cavity formedbetween the diaphragm, parts of the carrier metal layer and the carriersurface; a resonator metal layer including an interdigital transducer(IDT) formed on the front surface of the single-crystal piezoelectricplate, wherein interleaved fingers of the IDT are disposed on thediaphragm and over the cavity; and a second metal layer at selectedlocations over the surface of the resonator metal layer, wherein thesurfaces of the second metal layer are metal-to-metal bonded to surfacesof the carrier metal layer.
 15. The device of claim 14, wherein thepiezoelectric plate comprises openings extending through thepiezoelectric plate.
 16. The device of claim 15 further comprisingcontact bumps extending through the openings and bonding contact pads ofthe carrier wafer to contact pads of a capping layer.
 17. The device ofclaim 14, further comprising a passivation layer formed over theinterleaved fingers of the metal layer and disposed over the cavity. 18.The device of claim 14, wherein: the piezoelectric plate and theresonator metal layer are configured such that radio frequency signalsapplied to the resonator metal layer excites a primary shear acousticmode in the piezoelectric plate over the cavity, wherein a thickness ofthe diaphragm is selected to tune the primary shear acoustic modes inthe piezoelectric plate.
 19. The device of claim 14, wherein a thicknessof the plate ts is between 100 nm and 1000 nms.
 20. The device of claim14 wherein the contact pads of the second metal layer are metal-to-metalbonded to opposing contact pads of the carrier metal layer.